Introduction

The question of
whether or not the universe had a beginning is, of course, by no means an easy
question. When you ask a scientist a
question that is not easy, he never gives just one answer, but instead gives a
succession of answers. In this case, I
would like to offer two levels of answers.

At the first
level, I would argue that the answer to the question is yes, the universe had a
beginning in the event that is usually referred to as the big bang.

I
think that at least 99.9 percent of the people working in scientific cosmology
today believe that the universe evolved from a hot dense state, exactly as
Sandra Faber spoke about earlier. This
theory is strongly supported by the direct observation of the expansion of the
universe via the redshift of the light from distant galaxies, by the
measurement of the abundances of the light chemical elements, and by the now
very precise measurements of both the spectrum and the very small
nonuniformities of the cosmic microwave background radiation. Thus, most scientists (including me) believe
that the universe as we know it began in a big bang some 11 to 16 billion
years ago.

However,
as Sandra has already emphasized, there is another level to the question of
beginning. When cosmologists say that
they are persuaded that the big bang theory is valid, they are using a rather
precisely defined and restricted interpretation of the term "big
bang.'' As it is used by scientists the
term refers only to the expansion of the universe from an initially hot dense
state. But it says nothing about
whether the universe really began there, or whether there was something else
that preceded what we call the big bang.

So,
beyond the standard big bang, there is now a very significant body of research
concerning the possibility of cosmic inflation. Today I want to talk about inflation, and in
particular I want to talk about a very likely ramification called eternal
inflation. As you will see, the theory
of inflation does not give a clear answer to the question of whether the
universe had a beginning, but it does provide at least a context for discussing
this question.

To
begin, I would like to highlight the distinction between the questions that the
standard big bang theory answers and the questions that inflation is intended
to answer.

The
standard big bang theory is, of course, a very significant scientific
theory. It describes how the early
universe expanded and cooled from an initially very hot dense state. It describes how the light chemical elements
that we observe today were synthesized during the first 200 seconds or so of
this expansion period. And finally,
although work in this area is still in progress, it seems to describe very well
how the matter in the universe eventually congealed to form the stars,
galaxies, and clusters that we observe in the universe today.

There
is, however, a key issue that the standard big bang theory does not discuss at
all: it does not tell us what banged, why it banged, or what happened before it
banged. Despite its name, the big bang
theory does not describe the bang at all.
It is really only the theory of the aftermath
of a bang.

So,
in particular, the standard big bang theory does not address the question of
what caused the expansion; rather, the expansion of the universe is
incorporated into the equations of the theory as an assumption about the
initial state - the state of the universe when the theory begins its
description.

Similarly,
the standard big bang theory says nothing about where the matter in the
universe came from. In the standard big
bang theory all the matter that we see here, now, was already there, then. The matter was just very compressed, and in
a form that is somewhat different from its present state. The theory describes how the matter evolved
from one form to another as the universe evolved, but the theory does not
address the question of how the matter originated.

While
inflation does not go so far as to actually describe the ultimate origin of the
universe, it does attempt to provide a theory of the bang: a theory of what it
was that set the universe into expansion, and at the same time supplied
essentially all of the matter that we observe in the universe today.